Arc Forum | Saying (do.foo ...) instead of (foo ...) just makes it so that 'foo isn't in fun...

Arc Forum

3 points by rocketnia 4884 days ago | link | parent

Saying (do.foo ...) instead of (foo ...) just makes it so that 'foo isn't in function position. That means the local variable 'foo will always be used instead of some macro named 'foo that happened to be defined in another library.

The ssyntaxes .foo and !foo are short for get.foo and get!foo. Also, a:b ssyntax is handled before a.b ssyntax, giving the : lower precedence.

That means (.new-val:do.setter-getter) compiles as though it's ((get new-val) ((do setter-getter))), which makes it a roundabout way to accomplish ((do.setter-getter) new-val) with one less pair of parentheses, one less character, or (by my count) one less token.

It's really sort of silly in this case, since you save on so little while rearranging so much. In its favor, he (.a:b ...) idiom makes a bit more sense when it's a field/element access like (!width:get-rect ...) or (!2:setforms ...). It's especially helpful when it's part of a big chain of composed forms like (.a:b:.c:.d:e ...).

In my own code, including Lathe, I define this:

  (def call (x . args)
    (apply x args))

Then the code is just call.setter-getter.new-val, giving a savings of two characters, two pairs of parentheses, or (by my count) four tokens, all without causing the expressions to be rearranged.

Seems like a lot of brevity boilerplate, huh? :-p

2 points by aw 4884 days ago | link

I remembered talking about this once and found the comment: http://arclanguage.org/item?id=11697

It looks like it would be an easy change to Arc to have local variables take precedence, though I haven't tried the patch myself.

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1 point by akkartik 4884 days ago | link

Oh nice. So is there a way using this trick to do

  a."b".c.d.g

without parens?

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3 points by rocketnia 4884 days ago | link

Not in Arc. The best you get is (.g:.d:.c:a "b"), I think. You can shove "b" into ssyntax with string!b, but you need to use it as an argument somehow, and any ssyntax containing .string!b will pass the 'string function itself as an argument instead of calling it first.

Of course, if "b" were a symbol instead of a string, it would just be "a!b.c.d.g".

In Penknife, the answer to your question is actually yes: It's "q.b'a.c.d.g", where q is the string-quoting operator and ' is an operator that acts like a reverse a.b. It's easy to stump Penknife too, but I'm hoping to make a full thread about Penknife's take on infix operators in a couple of days or so.

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2 points by thaddeus 4881 days ago | link

Optionally, in ac.scm:

  (define (has-ssyntax-char? string i)
    (and (>= i 0)
         (or (let ((c (string-ref string i)))
               (or (eqv? c #\:) (eqv? c #\~) 
                   (eqv? c #\&)                
                   (eqv? c #\%)                
                   ;(eqv? c #\_) 
                   (eqv? c #\.)(eqv? c #\!)))
             (has-ssyntax-char? string (- i 1)))))

  (define (expand-ssyntax sym)
    ((cond ((or (insym? #\: sym) (insym? #\~ sym)) expand-compose)
           ((or (insym? #\. sym) (insym? #\! sym)(insym? #\% sym)) expand-sexpr)
           ((insym? #\& sym) expand-and)
       ;   ((insym? #\_ sym) expand-curry)
           (#t (error "Unknown ssyntax" sym)))
   sym))

  (define (build-sexpr toks orig)
    (cond ((null? toks)
           'get)	    
          ((null? (cdr toks))
           (chars->value (car toks)))
          (#t
           (list (build-sexpr (cddr toks) orig)
                 (cond ((eqv? (cadr toks) #\!)
                        (list 'quote (chars->value (car toks))))
                       ((eqv? (cadr toks) #\%)
                         (list 'string (list 'quote (chars->value (car toks)))))
                       ((or (eqv? (car toks) #\.)(eqv? (car toks) #\!)(eqv? (car toks) #\$)(eqv? (car toks) #\%))
                         (err "Bad ssyntax" orig))
                       (#t 
                       (chars->value (car toks))))))))

Then:

  arc> (= test (obj "a" "my a val" "b" "my b val"))
  #hash(("a" . "my a val") ("b" . "my b val"))

  arc> test%a
  "my a val"

[edit: actually, it would be nicer to have the percent symbol represent the spaces in the string and have some other symbol signify string handling, but I never got around to it + my scheme foo is lacking :)]

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